Saturable reactor current limiter

ABSTRACT

A current limiter to protect circuit elements from excessive current caused by transient load impedance or power supply surges. A multiple branch element magnetic circuit includes at least one saturable branch element inductively coupled by a coil connected in series with the load of an external electric circuit. The magnetic circuit further includes lower permeance bias branch element which biases the saturable branch element in a low permeance, saturated state when current through the coil is below a predetermined threshold value, and which biases at least one saturable branch element in its high permeance, non-saturated state when current through the coil is in a predetermined range above and extending from the threshold value. A relatively high permeance shunt branch element provides a shunt path for magnetic flux. The coil provides a relatively high inductive impedance in series with the external electrical circuit when at least one saturable element is in its non-saturated state and provides a relatively low impedance otherwise.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my U.S. patent applicationSer. No. 635,895, filed Nov. 28, 1975 now U.S. Pat. No. Pat. No.4,031,457, which is a continuation-in-part of my U.S. patent applicationSer. No. 614,773, fild Sept. 19, 1975, now abandoned. The presentapplication is assigned to the same assignee as Ser. No. 635,895 andSer. No. 614,773.

FIELD OF THE INVENTION

The present invention relates to current limiting devices and inparticular to a magnetically biased saturable reactor current limiter.

BACKGROUND OF THE INVENTION

There is a need to protect circuit elements from excessive currentcaused by transient load impedance or power supply surges. The commonlyapplied thermal fuse or electromagnetic circuit breaker opens thecircuit at a predetermined current or current-time product. Such devicesare slow acting and do not respond to fast acting transients. Inaddition, a fuse must be replaced after open circuiting and not allcircuit breakers close automatically after transient conditions.

Other approaches to limiting current have led to complex or weightycircuit elements.

SUMMARY OF THE INVENTION

In accordance with the present invention, a magnetically biasedsaturable reactor transient current limiter is provided that is fastacting, compact and automatically resets upon return to a normal currentlevel. When coupled in series with the load in an electrical circuit,the current limiter introduces relatively low inductive impedance inseries with the load when current through the circuit is below apredetermined threshold. However, when the current enters apredetermined range above that threshold, the device presents arelatively high inductive impedance in series with the load. The limiterautomatically resets to a low impedance upon return to current levelsbelow the threshold. The invention may be configured to limit direct andalternating (including three phase) currents.

According to one form of the invention, the current limiting devicecomprises at least three magnetic circuit branch elements extendingbetween two nodes, and an input coil for electrically coupling thedevice to an external electric circuit. The first of the branchelements, or shunt leg, is characterized by a relatively high permeancethroughout the range of operation of the device. The second of thebranch elements is composed of a saturable magnetic material and passesthrough the input coil so that magnetic flux is induced in the secondelement in response to current in the input coil. This second branchelement is characterized by a relatively high permeance when in itsnon-saturated state and a relatively low permeance otherwise. The thirdof the branch elements, or bias leg, is characterized by a relativelylow permeance and includes an mmf bias means for establishing arelatively constant magnetic flux in that element throughout the rangeof operation of the device.

The relative permeances of the three elements establish a net magneticflux density in the second element equal to the saturation flux densityfor that element when the current in the external electric circuit isbelow a predetermined threshold value. The net flux density in thatsecond element is below the saturation flux density when the current inthe input coil is in a predetermined range above and extending from thethreshold. The permeance of the first and second legs (whennon-saturated) is relatively high with respect to the medium external tothe device, and, in addition, the non-saturated permeance of the firstleg is less than the non-saturated permeance of the second leg. In thisconfiguration, the shunt leg provides a high permeance flux path toaccommodate the permeance changes in the second branch element when theinput current crosses the threshold value.

For limiting a.c. surges, the first element may be configured with apair of sub-elements extending between the nodes. In this configuration,the input coil includes windings which extend about each of thesub-elements and are connected so that the oppositely directed flux isinduced in the sub-elements in response to current in the coil.

The invention may also limit current surges in n-phase systems byutilizing additional branch elements with associated input coilsconfigured in the same manner described above for a.c. operation.

According to another form of the invention, the current limiting devicecomprises a multiple branch element magnetic circuit and an input coilfor electrically coupling the device to an external electric circuit.The magnetic circuit includes a magnetic bridge network having a pair ofupper leg elements joined at one end at a first node, a pair of lowerleg elements joined at one end at a second node, and a cross elementhaving each of its ends joined to a lower and upper leg element at athird and fourth node, respectively. The circuit further includes amagnetic bias element connected between the first and second nodes. Thecross element passes through the input coil so that magnetic flux isinduced in the cross element in response to current in the input coil.

The upper and lower legs connected to the third node are each composedof a saturable magnetic material and provide flux paths between thefirst and third and between the second and third nodes, respectively,which are characterized by a relatively high permeance when therespective leg elements are in their non-saturated states and arelatively low permeance otherwise.

The upper leg element connected to the fourth node (hereafter denoted asthe upper shunt leg), together with the cross element, and the lower legelement connected to the fourth node (hereafter denoted as the lowershunt leg), together with the cross element, provides a flux pathbetween the first and third nodes and between the second and thirdnodes, respectively, which are characterized by a relatively highpermeance throughout the range of operation of the device.

The bias element is characterized by a relatively low permeance, andincludes an mmf bias means for establishing a relatively constantmagnetic flux in that element throughout the range of operation of thedevice. For example, the bias may comprise a permanent magnet or,alternatively, a coil with an associated current source configured toproduce a magnetic field equivalent to that produced by such a permanentmagnet.

The relative permeances of the bridge elements establish a net magneticflux density in the upper and lower leg elements connected to the thirdnode equal to the saturation flux density for those elements when themagnitude of the current through input coil is below a predeterminedthreshold value in each polarity. The net flux density in the upperelement connected to the third node is below the saturation flux densitywhen the magnitude of the current in the input coil is in apredetermined range above and extending from one threshold value, whilethe lower element connected to the third node remains saturated.Similarly, the net flux density in the lower element connected to thethird node is below the saturation flux density when the magnitude ofthe current in the input coil is in a predetermined range above andextending from the other threshold value, while the upper elementconnected to the third node remains saturated. The permeance of thecross, upper and lower legs (when non-saturated) is relatively high withrespect to the medium external to the device, and, in addition, thepermeance of each of the upper and lower legs connected to the thirdnode when non-saturated is greater than the permeance of the seriescombination of the cross element with either the upper or lower shuntleg. In this configuration, the respective series combinations of thecross element with the upper and lower shunt legs provides a highpermeance flux path to accommodate the permeance changes in thecorresponding upper and lower leg elements connected to the third nodewhen the input current exceeds the threshold value. Accordingly, thisconfiguration of the present invention is suitable for limiting currentsurges in either d.c. or a.c. cases, since input current surges in onepolarity drive the upper leg element connected to the third node out ofsaturation, while input current surges in the other polarity drive thelower leg element connected to the third node out of saturation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the present invention are illustrated belowin the preferred embodiments, presented by way of example and notlimitation, and in the accompanying drawings of which:

FIG. 1 is a pictorial view of a current limiter showing an air gappedreactor biased by a permanent magnet;

FIG. 2 is a magnetic circuit diagram of the current limiter shown inFIG. 1;

FIG. 3 is a magnetization curve for the saturating leg of the magneticcore in FIG. 1 under conditions of normal and high transient currents;

FIG. 4 is a magnetization curve for the non-saturating leg of themagnetic core in FIG. 1;

FIG. 5 is a demagnetization curve for the permanent magnet as shown inFIG. 1;

FIG. 6 is a plot of current against time during a cycle of normal andhigh transient currents for the current limiter as shown in FIG. 1;

FIG. 7 is a current limiter wherein a section of low permeabilitymaterial is attached coextensively to the core;

FIGS. 8 and 9 illustrate current limiters with power dissipationcircuits for reactor stored energy;

FIG. 10 depicts a current limiter for alternating surge currents;

FIGS. 11-13 show additional embodiments of alternating current limiters;

FIG. 14 illustrates a current limiter in accordance with the presentinvention; and

FIG. 15 is an electric circuit analog of the current limiter shown inFIG. 14.

FIG. 16 illustrates an alternative embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment for a direct current limiter comprises amagnetic circuit having three branch elements (or legs) extendingbetween two nodes. The first leg is composed of a saturable material andis configured with an air gap. The second leg is also composed of asaturable material and is wound with an inductive coil which iselectrically connected in series with the load. In the useful range ofthe embodiment, the first leg is characterized by a permeance which isrelatively high compared to the medium external to the device, but lessthan the permeance associated with the second leg in its non-saturatedstate. The third leg is a permanent magnet. The winding direction of thecoil is such that current through the coil produces flux in the secondleg in opposition to the flux coupled therein from the permanent magnetleg. At relatively low coil current levels, the flux produced by thepermanent magnet overcomes that produced by the coil so that the netflux in the second leg is sufficient to drive that leg into saturationand thereby maintain a low inductance circuit in series with the load.The permanent magnet also establishes a relatively low level of flux inthe first leg.

With this configuration, a high transient current in the coil generatesa high magnetomotive force (MMF) tending to offset the permanentmagnet-induced flux in the second leg, thereby reducing the net flux inthat leg. The reduction in flux in the second leg is accompanied by arelated increase in flux in the third leg, and by a relatively smallflux change in the permanent magnet path. As the second leg is drivenout of saturation, its permeance is sharply increased. As a result, theinductive impedance provided by the coil to the external circuit isgreatly increased during transient current conditions. When the currentthrough the coil returns to a relatively low level, the above processreverses and the second leg returns to a permanent magnet inducedsaturated state.

The direct current limiter 10 as shown in FIG. 1 includes a core 16 ofsaturable magnetic material formed in a C-shape with a gap 20 (filledwith a relatively low permeability medium such as air) between planarsurfaces 14 and 15. A winding 17 of the input coil, (excited by loadcurrent I), is disposed about the portion of core 16 opposite gap 20. Apermanent magnet 13 is positioned inside core 16 to excite it in twoparallel magnetic flux circuits, one coupling winding 17 and the otherpassing through gap 20.

The magnetic circuit for current limiter 10 is shown schematically inFIG. 2. Magnetic branch elements (or legs) 21-23 extend between twonodes. Permanent magnet 13 is represented in leg 21 by its magnetomotiveforce (MMF) F_(M) serially connected to its permeance P_(M) andgenerating flux φ_(M). Current I is polarized with respect to terminals18 and 19 to generate MMF F_(L) in winding 17 in opposition to F_(M)resulting in magnetic flux φ_(L) in leg 22. P_(L) is the effectivepermeance of the partial section of core 16 through which flux φ_(L)passes. Leg 23 comprises P_(R), the resultant of the permeances of gap20 and the effective permeance of the connective core material formingthe gap through which flux φ_(R) branches off from the permanent magnet.The above description of the magnetic circuit is expressedmathematically as:

    φ.sub.M = φ.sub.L + φ.sub.R                    Eq. 1

    F.sub.M - (φ.sub.M /P.sub.M) - (φ.sub.L /P.sub.L) - F.sub.L = 0 Eq. 2

    F.sub.M - (φ.sub.M /P.sub.M) - (φ.sub.R /P.sub.R) = 0 Eq. 3

At normal current levels the magnet MMF (F_(M)) overcomes F_(L) togenerate a high level of flux φ_(L) to saturate the core material in leg22, creating a low permeance P_(L) and thus a low inductive impedance tocurrent flow. Also, the magnet sets a low unsaturated level of fluxφ_(R) in leg 23.

A high transient current drives leg 22 out of the saturation region ofits core material and into the high permeance region, thereby increasingthe inductive impedance of winding 17. Leg 23 provides a flux shuntingpath whereby the decrease in flux in leg 22 is balanced primarily by anincrease in leg 23 flux and a small decrease in leg 21 flux. In thismanner, the permanent magnet operates in its reversible magnetic regionthroughout normal and transient current conditions, preventingdemagnetization and allowing resetability of the current limiter uponreturn to normal current levels.

FIGS. 3, 4 and 5 further illustrate the operation of the invention.FIGS. 3 and 4 are magnetization curves of the core materials in legs 22and 23 respectively for various operating conditions, and FIG. 5 is thedemagnetization curve of the permanent magnet.

When no current is delivered to winding 17 the permanent magnet 13biases the core material of leg 21 deep into the saturation region ofthe hysteresis loop designated by reference numeral 31 as shown in FIG.3. As described, when current flows, the magnetomotive force is opposedto that of the magnet, thus moving the operating point to the portion ofthe curve designated by numeral 32. Increased current moves theoperating point to position 33 where the core begins to be driven out ofsaturation. When operating in the low permeance saturation region of itscore, winding 17 offers low inductive impedance to current. Anabnormally high transient current drives the core into the highpermeance region of section 34 of the curve where winding 17 exhibits ahigh inductive impedance, thereby opposing a further increase incurrent. Point 35 on the curve is reached at the peak of the transientcurrent. For resetability of the current limiter, the transient currentmay not exceed the value causing irreversible demagnetization of thepermanent magnet 13. As the current returns to normal levels, theoperating point moves through section 36 of the curve and into thesaturation region of section 32.

FIG. 5 is exemplary of a demagnetization curve for permanent magnet 13of an alnico type material. Increased winding current tends to increasethe MMF within the magnet driving the operation point down throughsection 51 of the curve. Driving the MMF beyond the knee 52 of the curvewould cause irreversible demagnetization. Samarium cobalt of the otherrare earth cobalt magnet materials known for their high resistance toirreversible demagnetization may be employed.

Preferably, the core material of leg 23 is operated at non-saturatingflux density levels throughout normal and transient current conditions,the air gap permeance configured to provide a near saturation conditionat the maximum transient current. As shown in FIG. 4, section 41 of thehysteresis loop represents increasing flux density in the core materialsof leg 23 induced by a high transient current increasing themagnetomotive force F_(L) in leg 22. Section 42 of the curve representsa closed loop return to the lower flux density at a normal currentcondition. Therefore, leg 23 provides the required shunting path for thecollapsing flux field in leg 22. when a high transient current isapplied to winding 17.

While the core may be made from any magnetically saturable material, itis preferred that it exhibit high permeability square loop hysteresischaracteristic such as that of manganese zinc ferrites or nickel ironalloys.

As an alternative to the use of a permanent magnet to form the thirdleg, a bias coil and associated means for driving a bias current throughthe bias coil may be utilized. In such a configuration, the flux path inthe third leg includes the region interior to the bias coil and therelatively low permeance air gaps between the bias coil and the junctionpoints of first and second legs. Accordingly, the bias coil establishesthe bias flux in the first leg. Current limiters for high currenttransmission lines may economically employ such a bias means usingcryogenically cooled super conducting windings, requiring little or nopower input to maintain a large biasing flux field.

Furthermore, various core configurations may be used so long as a coreis combined with a biasing means to provide the required saturated andshunt magnetic flux paths according to the invention. In general, thecore is configured and the magnet located to minimize leakage flux fromthe magnet.

Although it is generally desirable for the current limiter to exhibitlow inductance at normal input current levels, where higher inductanceis required, a second winding may be disposed on a non-saturatingsection of the core connected to the first winding in a series aidedrelationship. In this manner, the impedance to the current flow throughthe two windings is determined for both normal and high transientcurrents.

FIG. 6 shows a plot of winding current against time during a cycle ofnormal current conditions and a surge such as occurs in the momentaryshorting of a load in series with current limiter 10. The portion 61 ofthe plot is a period of no current flow through winding 17. At time t₁,the current limiter is excited to a normal level 62. A load shortcircuit at time t₂ causes a short surge in current, represented byportion 63, until time t₃ when the core starts to be driven out ofsaturation corresponding to the flux density operating point 33 of FIG.3. As the current continues to increase, as represented by the portion64 of the curve, its rate of increase is retarded by the high impedanceof the winding operating in the unsaturated region of its core. At timet₄, the short circuit is removed and current returns to a normal currentlevel 65. In this manner, a high transient current is prevented fromflowing in the circuit.

FIG. 7 depicts an alternate embodiment of the invention comprising ahigh permeability C-shaped core 71, a winding 73 disposed thereon, a lowpermeability I-shaped section 74 bridging the end faces of core 71 and apermanent magnet 72 biasing means. The equivalent permeance for leg 23of FIG. 2 is achieved by means of a gapless section of low permeability,high saturation magnetic material. The relationship of the variousparameters is such that core 71 is biased into saturation according tothe invention as previously discussed.

FIG. 8 is an embodiment of the invention having a core 81, permanentmagnet 82 and winding 83 similar to current limiter 10 of FIG. 1.Resistor 84 in series with diode 85 serves as a shunt circuit acrosswinding 83 to dissipate potentially harmful stored energy in the reactorafter a current surge. For example where a momentary load short causes acurrent surge, the peak current reached when the short is removedcontinues to flow in winding 83. Rather than allow the full current toflow through the load, it is partially shunted, thus dissipating energyin resistor 84 and diode 85. Since the voltage polarity across thewinding reverses when the short is removed, diode 85 blocks current flowbefore but not after removal of the short.

The embodiment of FIG. 9 shows an alternate arrangement for dissipatingstored energy in the reactor. Core 91, winding 93 and permanent magnet92 are configured and disposed in similar fashion to current limiter 10of FIG. 1. A transformer winding 94 is disposed on the unsaturatedsection of core 91, shunted by resistor 95 and diode 96 in series. Uponremoval of a load short circuit, the rate of change of flux in the corereverses from a positive to a negative rate, thus reversing the voltagepolarity across winding 94. The polarities of winding 94 and diode 96are determined to allow current flow and power dissipation in resistor95 after removal of the load short but not before.

The embodiment shown in FIG. 10 provides for limiting surges ofalternating current. A high permeability core 100 includes legs 101, 102and air gapped leg 103. Windings 105 and 106, disposed on legs 101 and102 respectively, are connected in a series opposed relationship.Permanent magnet 1014 biases legs 101 and 102 into saturation undernormal alternating current inputs to the windings, while the interlegconnective material in core 100 is dimensionally proportioned to operateat unsaturated levels for both normal and surge conditions. At anyinstant of time, one winding is driving its leg deeper into saturationwhile the other winding is opposing the MMF of the permanent magnet andtending to drive its leg out of saturation. The situation is reversedwith a change in current direction. Legs 101 and 102 are alternatelydriven out of saturation by an alternating current surge, leg 103serving as a flux shunt. In turn, windings 105 and 106 offer highimpedance to surging current flow according to the invention.

FIGS. 11 and 12 show another embodiment for limiting alternatingcurrent. Windings 113 and 114 are connected in series aidingrelationship on opposing legs 115 and 116 respectively of closed core111. Permanent magnet 110 is centered inside of the core 111 to biaslegs 115 and 116 into saturation under normal currents conditions, whiletheir interconnective legs are proportioned to operate in an unsaturatedcondition for normal and surge currents. A C-shaped core 112 centrallybridges core 111 in perpendicular alignment. An air gap is providedbetween face 117 of core 112 and face 118 of core 111. Core 112 isbiased by permanent magnet 110 at a low unsaturated level of flux andprovides a flux shunting path when legs 115 and 116 are alternatelydriven out of saturation by an alternating current surge.

As with the aforementioned embodiments, the permanent magnet (magnet110) may be replaced by a bias coil and associated means for providing abias current through the coil.

An alternative embodiment is shown in FIG. 13 for a three phase currentlimiting device having three pairs of wound legs 121 and 122, 123 and124, 125 and 126, respectively, common shunting leg 130 and a bias leg132, all extending between two nodes. Each of the three pairs have anassociated input coil wound in a similar manner to the pair of legsdescribed above in conjunction with FIGS. 11 and 12. The bias leg 132may comprise a permanent magnet or a bias coil and associated currentsource. The shunt leg 130 in the present embodiment is a C-shaped coresimilar to core 112 in FIGS. 11 and 12, establishing an air gap betweenthe upper portion of shunt leg 130 and the upper junction of the pairsof wound legs 121-126 and bias leg 132. In operation, with relativelylow currents in the windings, bias leg 132 saturates legs 121-126 whilemaintaining shunt leg 130 in its non-saturated state. Alternatingcurrent surges in the windings alternately drive the associated legs outof saturation in a similar manner to the previously describedembodiments. In n-phase systems, n pairs of legs and associated inputcoils may be similarly arranged. In the multiple phase embodiments, theinput coils are connected to the external circuit as in a three phasemotor. Circuitry for dissipation of stored energy, such as varistor, maybe connected across each leg winding.

A single winding current limiting device 200 for a.c. or d.c. lines isshown in FIG. 14. An electrical circuit analog configuration for thisdevice is shown in FIG. 15. The device 200 includes an input coil 301having terminals 203 and 205 for electrically coupling the device 200 toan external electric circuit. The magnetic circuit includes a bridgenetwork having a pair of upper leg elements 207 and 209 joined at afirst node, N1, a pair of lower leg elements 211 and 213 joined at oneend at a second node, N2, and a cross element 215 having a first endconnected to legs 207 and 211 at a third node, N3, and its other endconnected to legs 209 and 213 at a fourth node, N4. The cross element215 passes through coil 201 so that magnetic flux is induced in thecross element in response to current in the input coil 201.

In the illustrated embodiment, each of legs 209 and 213 include a highpermeability, solid portion and a respective one of air gaps 209a and213a, with the gaps between the respective solid portions and the crosselement 215. In this embodiment, the gap portions of legs 209 and 213and the end of leg 215 are defined as "joined" at N4. In otherconfigurations, for example, the legs 209 and 213 may include solidportions physically joined to the cross element 215 at N4 with the gaps209a and 213a located at intermediate points along legs 209 and 213,i.e. between nodes N1 and N4, and N2 and N4, respectively.Alternatively, the relative permeances associated with legs 209, 213 and215 may be controlled by the geometry and material composition so thatgaps may not be necessary.

The device 200 further includes a magnetic bias element 217(characterized by permeance P_(E)) connected between nodes N1 and N2. Inthe illustrated embodiment, the bias element 217 is a permanent magnethaving a north pole near node N1 and a south pole near node N2. Inalternative embodiments, a suitably arranged coil and associated currentbias source may establish equivalent fields at the nodes N1 and N2, andmay be used in lieu of permanent magnet 217.

In the present embodiment, legs 207 and 211 are each composed of asaturable magnetic material and thus, these legs provide flux pathsbetween N1 and N3 and between N3 and N2 which are characterized bypermeances (P_(A) and P_(B)) which are relatively high when therespective leg elements are in their non-saturated states, andrelatively low otherwise.

The upper leg element 209 and the associated air gap 209a together withthe cross element 215 provide a shunt path between nodes N1 and N3characterized by a relatively high permeance (P_(C)) throughout therange of operation of the device. Similarly, leg element 213 at itsassociated gap 213a together with cross element 215 provide a flux pathbetween the nodes N2 and N3 which is characterized by a relatively highpermeance (P_(D)) throughout the range of operation of the device.

The electrical circuit analog shown in FIG. 15 corresponds to theconfiguration of FIG. 14, wherein each element is characterized by aconductance (analogous to the permeance) denoted by the referencedesignation P and an identifying numerical subscript. The voltage sourceV_(I) is representative of the flux induced in element 215 as a resultof current through coil 201, and the voltage source V_(BIAS) isrepresentative of the mmf bias associated with bias element 217.

The relative permeances of the bridge elements establish a net magneticflux density in both legs 207 and 211 equal to the saturation fluxdensity for those elements when the current in the external circuit(i.e. in coil 201) is between lower and upper threshold values of thatcurrent. When the current in coil 201 is in a predetermined range aboveand extending from the upper threshold value, the flux produced byelement 217 passes from node N1 by way of leg 209, cross element 215,leg 211 to node N2. In this case, leg 211 remains saturated while leg207 is in its non-saturated high permeance state. Similarly, whencurrent in coil 201 passes in the other direction and is in apredetermined range below and extending from the lower threshold value,the flux from bias element 217 passes from node N1 by way of leg 207,cross element 215 and leg 213 to node N2. In this case, the leg 207remains saturated while the leg 211 is in its non-saturated, highpermeance state.

Accordingly, excursions of current in coil 201 extending beyond theupper or lower threshold values require the external circuit to drivethe flux in device 200 by way of one of the air gaps 209a or 213a,depending on the polarity of the current surge in the coil 201. As aresult, at the point where the current in coil 201 exceeds thethreshold, one of the legs 207 and 211 switches from its saturated toits non-saturated state and the impedance presented by device 200 at theterminals 203-205 switches from a relatively low value to a high value,thereby providing a current limiting action in the external line.

A two windind current limiting device 290 for a.c. or d.c. lines isshown in FIG. 16. The device 290 includes input coils 292 and 294coupled in series between terminals 303 and 305 for electricallycoupling the device 290 to an external electric circuit. The magneticcircuit includes a bridge network similar to that shown in FIG. 14 andhaving a pair of upper leg elements 307 and 309 joined at a first node,N1, a pair of lower leg elements 311 and 313 joined at one end at asecond node, N2, and a cross element 315 having a first end connected tolegs 307 and 311 at a third node, N3, and its other end connected tolegs 309 and 313 at a fourth node, N4. The elements 307 and 311 passthrough input coils 292 and 294 so that mmf is induced across thoseelements in response to current in the respective coils.

In the illustrated embodiment, each of legs 309 and 313 include a highpermeability, solid portion and a respective one of air gaps 309a and313a, with the gaps between the respective solid portions and the crosselement 315. In this embodiment, the gap portions of legs 309 and 313and the end of leg 315 are defined as "joined" at N4. In otherconfigurations, for example, the legs 309 and 313 may include solidportions physically joined to the cross element 315 at N4 with the gaps309a and 313a located at intermediate points along legs 309 and 313,i.e. between nodes N1 and N4, and N2 and N4, respectively.Alternatively, the relative permeances associated with legs 309, 313 and315 may be controlled by the geometry and material composition so thatgaps may not be necessary.

The device 290 further includes a magnetic bias element 317(characterized by permeance P_(E)) connected between nodes N1 and N2. Inthe illustrated embodiment, the bias element 317 is a permanent magnethaving a north pole near node N1 and a south pole near node N2. Inalternative embodiments, a suitably arranged coil and associated currentbias source may establish equivalent fields at the nodes N1 and N2, andmay be used in lieu of permanent magnet 317.

In the present embodiment, legs 307 and 311 are each composed of asaturable magnetic material and thus, these legs provide flux pathsbetween N1 and N3 and between N3 and N2 which are characterized bypermeances (P_(A) and P_(B)) which are relatively high when therespective leg elements are in their non-saturated states, andrelatively low otherwise.

The upper leg element 309 and the associated air gap 309a together withthe cross element 315 provide a shunt path between nodes N1 and N3characterized by a relatively high permeance (P_(C)) throughout therange of operation of the device. Similarly, leg element 313 at itsassociated gap 313a together with cross element 315 provide a flux pathbetween the nodes N2 and N3 which is characterized by a relatively highpermeance (P_(D)) throughout the range of operation of the device.

The relative permeances of the bridge elements establish a net magneticflux density in both legs 307 and 311 equal to the saturation fluxdensity for those elements when the current in the external circuit(i.e. in coils 292 and 294) is less than a first threshold value in afirst polarity and less than a second threshold value in the otherpolarity. When a current of the first polarity in coils 292 and 294 hasa magnitude in a predetermined range greater than and extending from thefirst threshold value, the flux produced by element 317 passes from nodeN1 by way of leg 309, cross element 315, leg 311 to node N2. In thiscase, leg 311 remains saturated while leg 307 is in its non-saturatedhigh permeance state. Similarly, when a current of the other polarity incoils 292 and 294 has a magnitude in a predetermined range greater thanand extending from the second threshold value, the flux from biaselement 317 passes from node N1 by way of leg 307, cross element 315,and leg 313 to node N2. In this case, the leg 307 remains saturatedwhile the leg 311 is its non-saturated, high permeance state.

Accordingly, excursions in magnitude of the current in coils 292 and 294extending beyond the threshold values require the external circuit todrive the flux in device 290 by way of one of the air gaps 309a or 313a,depending on the polarity of the current surge in the coils 292 and 294.As a result, at the point where the current in coils 292 and 294 exceedsthe threshold, one of the legs 307 and 311 switches from its saturatedto its non-saturated state and the impedance presented by device 290across the terminals 303 and 305 switches from a relatively low value toa high value, thereby providing a current limiting action in theexternal line.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

I claim:
 1. A device for limiting current in an electrical circuitcomprising:A. a multiple branch element magnetic circuit including:i. abridge network having at least four nodes and a pair of upper legelements joined at one end at a first of said nodes, a pair of lower legelements joined at one end at a second of said nodes, and a crosselement having one of its ends joined to one of said lower and upper legelements at a third node, said cross element having the other of itsends joined to the other lower and upper leg elements at a fourthnode,wherein said upper and lower leg elements joining said third nodeare characterized by non-linear permeances P_(A) and P_(B),respectively, and have saturated and non-saturated states, P_(A) andP_(B) being greater when the corresponding one of said third node legelements is in its non-saturated state than when in its saturated state,and the magnetic flux density in one of said third node leg elementsbeing equal to a saturation flux density which is relatively invarientwith respect to magneto-motive force applied across that element when insaid saturated state and said magnetic flux density being less than saidsaturated flux density otherwise, and wherein the flux path between saidfirst and third nodes along said cross-element and said upper legelement joining said fourth nodes is characterized by a permeance P_(C),and the flux path between said second and third nodes along said crosselement and said lower leg element joining said fourth node ischaracterized by a permeance P_(D), ii. a magnetic bias elementconnected between said first and second nodes, said bias element beingcharacterized by a permeance P_(E) and including a magneto-motive force(mmf) bias means for establishing a substantially constant magnetic fluxdensity in said bias element, and B. an input coil including means forelectrically coupling said device to said electrical circuit so thatsaid current passes therethrough, said coil having a plurality ofwindings extending about said upper and lower leg elements joining saidthird node so that mmf is induced across those elements in response tosaid current, said windings being adapted so that said mmf is directedaway from said third node when said current is one polarity and isdirected toward said third node when said current is the otherpolarity,wherein said elements are magnetically coupled at said nodes,and wherein P_(C) and P_(D) are relatively high with respect to themedium external to said device, P_(A) and P_(B) are greater than P_(C)and P_(D) and are relatively high with respect to the medium external tosaid device when the respective third node leg elements arenon-saturated, and P_(E) is relatively low with respect to P_(C) andP_(D) and with respect to P_(A) and P_(B) when the respective third nodeleg elements are non-saturated, wherein further the net magnetic fluxdensity in one of said third node leg elements is equal to thesaturation flux density for that element when said current is of a firstpolarity and has a magnitude less than a first threshold value or whensaid current is of the other polarity, and said net flux density isbelow said saturation flux density when said current is of said firstpolarity and has a magnitude in a predetermined range greater than andextending from said first threshold value, and wherein further the netmagnetic flux density in the other of said third node leg elements isequal to the saturation flux density for that element when said currentis of said first polarity, or when said current is of said otherpolarity and has a magnitude less than a second threshold value, andsaid net flux density is below said saturation flux density when saidcurrent is of said second polarity and has a magnitude in apredetermined range greater than and extending from said secondthreshold value.